1. Field of the Invention
The present invention relates to an M-type antenna for vehicles used in traffic information communication systems, and more particularly, to an improvement in the power supply means of such an antenna.
2. Prior Art
One of the traffic information communication systems currently used is a so-called on-road wireless communication information system.
In this communication system, on-road stations are established at certain intervals along the road, and a limited-radius wireless communication zone is created by each one of the on-road stations. From these stations, non-variable information regarding the driver's current location, the road configuration, etc., as well as variable information regarding traffic conditions moving forward and road construction, etc., are communicated in a short period of time to each car passing through the limited-radius wireless communication zone.
Since this system provides information intermittently in the limited-radius wireless communication zones, it is not possible to provide continuous and detailed information as seen in the general wireless communication systems. However, the advantage of the wireless communication system is a secure and stable propagation path and a lack of interference with the radio waves. Thus, various kinds of information is supplied to the passing cars very quickly and accurately.
Incidentally, with regard to the on-road stations, there are two different kinds: independent on-road stations and on-line on-road stations. Though the independent on-road stations continuously provide non-variable information only, the on-line on-road stations can provide variable information sent from an information center through a wired or wireless circuit simultaneously with the non-variable information.
FIG. 4 shows an electrical diagram of a conventional M-type car antenna used in a traffic information communication system as described above.
As shown in this Figure, an antenna element 40 of the M-type car antenna has a pair of emission sections 41 and 42 with one end of each one of the sections being grounded. The emission sections 41 and 42 are vertical segments of a conductive material formed in an inverted U shape. The emission sections 41 and 42 are connected to each other in series via transmission sections 43 and 44. Another emission section 45, which works also as a power supply member, is connected to a connection point between the transmission sections 43 and 44.
The total length of the conductive material of the antenna element 40, such a length being from one emission section 41 to the other emission section 42 through the transmission sections 43 and 44, is set to be equal to the wavelength .lambda. of the radio wave used in the information communication system. In this antenna element 40, in order to ensure an effective emission, the direction of the electric current i1 and i2 in the emission sections 41 and 42, respectively, is aligned with the direction of the electric current i5 of the emission section 45, which works also as a power supply member.
As generally known, in an M-type antenna, unless the maximum electric current is provided in the same direction to the three vertical sections, i.e., the emission sections 41, 42 and 45 of the antenna element 40, the emission and reception of the radio waves does not take place.
FIG. 4(b) shows the characteristic of current distribution i and the characteristic of voltage distribution V of the antenna element 40 as described above. FIG. 4(c) shows the impedance characteristic Zi of the antenna element 40. The horizontal axes in FIGS. 4(b) and 4(c) represent the conductive paths of the antenna element 40, one path for the connecting points a1, b1, c1 and e, and the other for connecting the points e, c2, b2 and a2, which are presented as straight lines in the drawing for the sake of convenience.
The points a1 and a2 are both grounded and electrically short-circuited. Therefore, the electric current level at these points is at a maximum. Because the length between the points a1 and a2 is .lambda., a maximum electric current point is also created at point C, which is the center point between the points a1 and a2. Voltage has a right-angle phase differential relative to electric current. Therefore, at points a1, C and a2, where the electric current level is at the maximum, the voltage level is zero. At points b1 and b2, where the electric current level is zero, the voltage is at the maximum.
Impedance is zero at points a1, C and a2, where the electric current level is at the maximum and the voltage level is zero. At points b1 and b2, where the electric current level is zero and the voltage level is at the maximum, impedance is at the maximum (infinite). Accordingly, the power supply should take place at the point where the impedance is 50 ohms, which is on the path where the impedance changes from zero to maximum (infinite). This power supply point e is, as seen from the above explanation, slightly to the right or left of the center point C between the points a1 and a2.
In order to obtain effective emission using the conventional M-type antenna, the direction of the electric current i1 and i2 in the emission sections 41 and 42 at both sides must be the same as that of the electric current i5 of the emission section 45, which works also as a power supply member. Accordingly, it is necessary to set the length of the conductive path connecting points a1, b1, c1 and e, as well as that of the connecting points e, c2, b2 and a2, at .lambda./2. As a result, the entire length of the conductive path connecting points a1, b1, cl, c2, b2 and a2 would range between .lambda./2 and .lambda.. Thus, the conventional M-type antenna has a drawback in that it is quite large in size.
On the other hand, impedance rapidly changes from zero to infinite when proceeding from point C to b1 or b2. Because of this, it is very difficult to find a point where the impedance is exactly 50 ohms between the points C and b1 and between the points C and b2. Therefore, the conventional M-type antenna has problems in that the required precision is not easily obtained in mass production and that there are large variations in the antenna characteristics.